1. Reboot your machine and when Xubuntu is about to start hit ESC (or the key set to show the menu options at boot). You should see about 3 to 5 entries where one of them should be a older version of your core. Choose the right version and you are done.

Now if you don’t see the right core, then this is what you need to do:

1. Using Synaptic install a linux-image (use linux-image to search in synaptic) that reflect the right core that you want to downgrade. Take note on the information about the core. Ex.: 2.6.31-14-generic

2. Open your menu list at your Grub boot menu and see what options do you have to start your Xubuntu (use terminal).

mousepad /boot/grub/menu.lst

Now add the new menu core installed following the same format as the other ones.

Remember to always have your files backup. No matter if you are on Windows, Linux or Mac, a full recovery can always brings unexpected results (even knowing that on Linux is mostly likely to have a smooth transition).